Optical systems are often used in situations in which the ambient temperature of the immediate surroundings and thus of the system itself is not constant or is different from that at which the system was calibrated. One result of the changes in temperature is that the index of refraction of the optical elements of the system changes. Also the optical elements and particularly the support members that hold the elements in position relative to each other expand, contract and sometimes twist or are otherwise distorted. The cumulative effect of the thermally induced changes for each of the individual optical and mechanical elements is to cause a change in the effective focal length of the system and a shifting of the image on the detector. The change in effective focal length of the system is physically manifested as an expansion or shrinkage of the image size on the detector. The shifting of the image is known as the boresight phenomena, which is manifested as translation of the image on the face of the detector.
A typical example of an optical system for which the invention is intended is an optical system mounted on an airplane, missile, spacecraft, or satellite. All of these platforms operate in extreme environmental situations in which ambient temperature in which the optical system functions can be very different from that in the factory or laboratory in which the optical system was assembled and calibrated. Furthermore the ambient temperature can change rapidly and over a large range of values as a function of time. It is clear that in order to be able to accurately relate the information contained in the images obtained from the optical system with the actual scene being viewed it is necessary to correct for the thermal induced changes in boresight and effective focal length.
Conventional solutions to focal length drift in optical systems, particularly to temperature-induced focal length drift, generally involve the application of multiple lens and/or mirror arrays and/or electro-mechanical assemblies. One method is to introduce one or more temperature sensors, for example a thermocouple, into the system. Electro-mechanical assemblies are provided to move individual elements of the optical assembly in response to signals from the temperature sensor indicating that a change in temperature and/or in the temperature gradient over the optical element has taken place. Other methods rely on choosing the materials of which the optical elements are made so the temperature induced effects are cancelled out. A third approach is to electronically compensate for the change in effective focal length by using a temperature sensor to measure an average temperature of the elements of the optical sensor and to use the measured temperature to influence the way in which the signals that make up the detected image are processed.
An example of an electronic effective focal length compensator is disclosed in U.S. Pat. No. 5,663,563. The optical system is of a thermal imaging unit, which comprises optics that channels electromagnetic energy representing information from a distant scene onto a scanner that reflects the energy to an imager assembly that images the scene onto a detector assembly. The imager assembly includes a temperature sensor for sensing the imager assembly lens temperature. The output of the detector elements is the input to a readout integrated circuit that is part of the electronics associated with the detector assembly. The readout integrated circuit includes a time delay and integration clock, a multiplexer, and another clock for the multiplexer. The compensator includes a processor unit for controlling the various processes that take place in order to gather, process, and display the images. According to the teachings of U.S. Pat. No. 5,663,563, the effective focal length is compensated for by means of instructions programmed into the processor. These instructions allow the processor to control the rate of the multiplexer clock according to the temperature changes measured by the temperature sensor.
It is an object of the present invention to provide a method and system for compensating for thermally induced effective focal length and boresight changes that does not require accurate and constant temperature measurements.
It is another object of the present invention to provide a method and system for compensating for thermally induced effective focal length and boresight changes that does not require complicated calibration functions.
It is an object of the present invention to provide a method and system, which can be incorporated into various applications and optical systems operating at different electromagnetic wavelength ranges, to compensate for thermally induced effective focal length and boresight changes that.
Other characteristics and advantages of the invention will become apparent as the description proceeds.